1. Field of the Invention
The present invention relates to devices for detecting the speed of moving surfaces and, in particular, relates to apparatus for sensing variations in the rotational speed of an internal combustion engine.
2. Description of the Prior Art
The present invention is an improvement of the contact tachometer disclosed in copending U.S. application Ser. No. 604,191, filed Oct. 29, 1990 and entitled "Method and Apparatus for Determining Relative Contributions of Individual Cylinders of Internal Combustion Engines Using Contact Tachometer", now U.S. Pat. No. 5,182,512. That tachometer is a hand-held device which includes a hollow handle provided with a clevis at one end rotatably carrying a wheel having a plurality of openings therethrough.
The aforementioned copending application discloses the use of such a contact tachometer in analysis of cylinder power balance, i.e., the relative power contributions of individual cylinders of a multi-cylinder internal combustion engine. This technique takes advantage of the fact that the speed variation of the engine within a single engine cycle can give an indication of the cylinder power balance. Thus, in normal operation, each time a cylinder ignition event occurs, the power stroke of that piston causes the engine to accelerate momentarily. The increase in engine speed reaches a peak and then begins to decrease as the engine "coasts" until the ignition event for the next cylinder in the firing order. Thus, in a normally operating engine, a plot of the engine speed against time over a single engine cycle is in the shape of a generally sinusoidal waveform with one period or cycle of the waveform for each cylinder. If all cylinders are contributing equally to the overall engine power, the peaks of this waveform should all be at substantially the same height.
The contact tachometer disclosed in the aforementioned copending application detects variations in the speed of the engine over a single engine cycle by detecting variations in the speed of a linearly moving part of the engine, such as a fan belt. When the wheel is held against a moving surface of an engine, such as a fan belt or the like, the wheel is rotated, causing the openings to pass between an aligned light source and photocell for producing an alternating output signal, the frequency of which is proportional to the speed of the engine. The handle encloses circuitry which is connected to the photoelectric sensing means for powering it and for generating, in response to its output, a voltage level suitable for being input to an engine analyzer.
But such a photoelectric-type contact tachometer may be subject to inaccuracy or failure if the light source or photoelectric sensor becomes dirty, or if any of the openings in the contact wheel become clogged or partially obscured with dirt, grease or the like. The risk of such inaccuracy or failure is real, since vehicle engine compartments, where the tachometer is intended to be used, are inherently dirty.
It is known to utilize electromagnetic sensors for detecting engine speed variations, such a system being disclosed, for example, in U.S. Pat. No. 4,539,841. In such prior systems, the electromagnetic sensor is positioned adjacent to the teeth on the engine flywheel or ring gear. The sensor senses the passing of the teeth of the ring gear as it is rotated. While this system permits a fairly accurate and reliable indication of engine speed, it has the disadvantage that it requires measurement of the absolute engine speed. Furthermore, the installation of the sensor is very cumbersome and time consuming. The ring gear is relatively inaccessible in most engines and the installation of the magnetic sensor is critical. It must be very precisely positioned relative to the gear teeth and this position must be accurately maintained in order for the system to work properly.